1. Field of the Invention
The present invention relates to a method for increasing the yield of crops and to a yield-increasing agent for crops used in the above method. More particularly, the present invention relates to a method for increasing the yield of crops by treating a plant capable of yielding a crop with brassinolide under specific conditions and to a yield-increasing agent for crops which contains brassinolide as an active ingredient thereof.
2. Description of the Prior Arts
Hitherto, a number of compounds have been found as substances capable of controlling growth and propagation of plants. Brassinolide, one of such compounds, is a steroidal plant-growth regulator isolated in 1979 from pollen of Brassica napus L. and determined as (2.alpha.,3.alpha.,22R,23R)-tetrahydroxy-24S-methyl-B-homo-7-oxa-5.alpha.- cholestan-6-one having the following structure [Nature, Vol. 281, pp. 216-217(1979)]: ##STR2##
A great number of steroidal compounds are already known as hormones for animals and insects, but brassinolide is a steroidal compound found for the first time as a substance exhibiting physiological activities to plants. Thus, brassinolide is considered to be the 6th plant hormone subsequent to ethylene, auxin, gibberellin, cytokinin and abscicic acid, and is still being studied for its distribution in plants and its specific functions from the academic point of view.
Concerning the physiological effects of brassinolide to plants, various kinds of bioassay including the second internode elongation bioassay for kidney bean (Phaseolus vulgaris), rice lamina inclination bioassay and raphanus bioassay for radish (Raphanus sativus) have been made heretofore for comparison with other plant hormones [The Society for Chemical Regulation of Plants, Japan, 18(No. 1), 38-54(1983)]. As a result of gathering the previous reports, brassinolide is now being watched as possessing unique activities different from those seen in other plant hormones. Further, brassinolide exhibits a strong synergistic effect with auxin to various elongation bioassays for plants and also a synergistic effect with cytokinin to a propagation test for callus, thus proving significantly unique effects which will hardly be observed in using such known hormone alone. With respect to distribution of brassinolide in the plant kingdom, more than ten kinds of brassinolide analogues are already discovered widely in various plants in addition to brassinolide itself, and it is confirmed experimentally that these brassinolide compounds are contained widely in the higher plants such as rice (Oryza sativa), kidney bean (Phaseolus vulgaris), Chinese cabbage (Brassica pekinensis), tea (Thea sinensis), chestnut (Castanea spp.), hyacinth bean (Dolichos lablab), pine (Pinus thunbergii), cattail (Typha latifolia) and Distylium racemosum.
In the past, the effect of brassinolide to plants is known in the case of kidney bean wherein brassins as a crude extract from pollen of Rape (Brassica napus L.) is used [J. W. Mitchel and L. E. Gregory, Nature New Biology, 239, 253 (1972)] and in the case of radish, lettuce, kidney bean, pepper (Piper nigrum) and potato (Solanum tuberosum) wherein brassinosteroids as synthetic analogous compounds are used [Science, Vol. 212 (1981), 33-34]. The treatment for plants disclosed in these literatures comprises applying a lanolin paste to seedlings of kidney bean in case of the brassins and spraying an aqueous solution over seedlings in case of the brassionosteroids.
In Japanese Laid-open Patent Appln. No. Sho. 57-118503, there is disclosed 2R, 3S, 22S, 23S-tetrahydroxy-24S-ethyl-22S, 23S-5.alpha.-cholestan-6-one(22S, 23S-homobrassinolide), and their derivatives, one of the synthetic brassinolide analogues, which are explained therein as a substance effective for accelerating the growth of tomato (Lycopersicon esculentum), carrot (Daucus carota), mung bean (Phaseolus aureus), radish (Raphanus sativus), cucumber (Cucumis sativus) and azuki bean (Phaseolus angularis) by dipping seeds or seedlings of these plants in a solution of this 22S, 23S-homobrassinolide prior to soil culture. This reference further discloses that when tubers of potato, sweet potato seedlings, cuttings of branches of tea plant and seeds of tabacco are dipped in a solution of the 22S, 23S-homobrassinolide prior to cultivation, the 22S, 23S-homobrassinolide exhibits a growth-accelerating effect and that when the 22S, 23S-homobrassinolide is sprayed over fruit trees at the stage of anthesis, the diameter and weight of the fruits become larger. In Japanese Laid-open Patent Appln. No. Sho. 58-90578, there are also disclosed new synthetic 2R, 3S, 22R, 23R-tetrahydroxy-24S-ethyl-5.alpha.-cholestan-6-one and their derivatives (22R, 23R-homobrassinolide derivatives) and their use for accelerating the growth of various plants and improving the quality of agricultural products. In this reference, an elongation test for azuki bean is carried out by treating the seed with the 22R, 23R-homobrassinolide derivatives just before or after germination, but not concrete disclosure is given therein how the yield of crops is increased.
In general, plant-growth regulating agents are utilized by artificially controlling the growth of plants to achieve increased yields of crops, regulation of the amount of the agricultural products, improvement in quality, saving of work time and power, and regulation of harvest time. These plant-growth regulating agents are comprised chiefly of plant hormones, synthetic compounds possessing activities equivalent to such plant hormones and chemical substances having antagonistic effects to these. Recognized now as plant hormones are auxin, gibberellin, cytokinin, abscicic acid and ethylene. Many of the chemical substances actually employed for agricultural use a plant-growth regulating agents possess activity similar to those five plant hormones.
On the other hand, a great number of compounds are known which show physiological activity in vitro bioassay to plants, but the number of the compounds actually utilized for practical use are rather small. In ordinary in vitro bioassay, its experimental system is simplified with a view to detecting only a particular reaction, sharply minimizing the mutual effect with other organs and tissues. However, a plant body contains in its body a plurality of organs which are different in age and function and grow under their mutual actions to keep coordination. Thus, it is rather rare that the activity observed in vitro bioassay is reproduced exactly in whole plants. Further, strength of in vitro bioassay does not respond to importance in practical use. It is often observed that substances which exhibit similar equilivant activity in vitro bioassay may exhibit quite different activity to cultivated plants. In addition, it is also known that a quite different activity is often exhibited according to the concentration of the compound used. Furthermore, it is usual that the growing phenomena observed in applying the growth-accelerating agent to plants are different according to the sort and age of the plants. Thus, the growth reaction of plants to foreign substance varies according to the sort of plants and to the stage of their growth. In the extreme case, plants may often show a counter-reaction to such growth-increasing agent. It is quite impossible, therefore, to estimate growth-regulating activity on the basis of its in vitro bioassay. Thus, development of a new plant-growth regulating agent always encounters a great difficulty and is only possible by repeating "trial and error" tests to check its practical usefulness.
All of the prior art methods hitherto known relate to tests wherein a crude extract containing brassinolide or a synthetic brassinolide analogue is exclusively used for accelerating the growth of plants, but fail to disclose the effect of pure brassinolide itself on the growth of plants. Under these circumstances, therefore, there was still a great demand for developing a new method of using brassinolide for accelerating the growth of plants, especially for increasing the yield of crops.